The present invention relates to an apparatus for, and the processing of, semiconductor substrates. In particular, the invention relates to the patterning of thin films during substrate processing.
Since semiconductor devices were first introduced several decades ago, device geometries have decreased dramatically in size. During that time, integrated circuits have generally followed the two year/half-size rule (often called Moore""s Law), meaning that the number of devices that will fit on a chip doubles every two years. Today""s semiconductor fabrication plants routinely produce devices with feature sizes of 0.5 xcexcm or even 0.35 xcexcm, and tomorrow""s plants will be producing devices with even smaller feature sizes.
A common step in the fabrication of such devices is the formation of a thin film on a substrate by chemical reaction of gases. When patterning thin films, it is desirable that fluctuations in line width and other critical dimensions be minimized. Errors in these dimensions can result in variations in device characteristics or open-/short-circuited devices, thereby adversely affecting device yield. Thus, as feature sizes decrease, structures must be fabricated with greater accuracy. As a result, some manufacturers now require that variations in the dimensional accuracy of patterning operations be held to within 5% of the dimensions specified by the designer.
Thin films are often patterned by etching away portions of the deposited layer. Modern substrate processing systems often employ photolithographic techniques in the patterning process. Typically, such photolithographic techniques employ photoresist or other light-sensitive material. In conventional processing, photoresist is first deposited on a substrate. A photomask (also known simply as a mask) having transparent and opaque regions that embody the desired pattern is positioned over the photoresist. When the mask is exposed to light, the transparent portions permit the exposure of the photoresist in those regions, but not in the regions where the mask is opaque. The light causes a chemical reaction in exposed portions of the photoresist. A suitable chemical, chemical vapor or ion bombardment process is then used to selectively attack either the reacted or unreacted portions of the photoresist. This process is known as developing the photoresist. With the remaining photoresist acting as a mask, the underlying layer may then undergo further processing. For example, material may be deposited, the underlying layer may be etched or other processing carried out.
Modern photolithographic techniques often involve the use of equipment known as steppers, which are used to mask and expose photoresist layers. Steppers often use monochromatic (single-wavelength) light, enabling them to produce the detailed patterns required in the fabrication of fine geometry devices. As a substrate is processed, however, the topology of the substrate""s upper surface becomes progressively less planar. This uneven topology can cause reflection and refraction of the monochromatic light, resulting in exposure of some of the photoresist beneath the opaque portions of the mask. As a result, this uneven surface topology can alter the patterns transferred by the photoresist layer, thereby altering critical dimensions of the structures fabricated.
Reflections from the underlying layer also may cause a phenomenon known as standing waves. When a photoresist layer is deposited on a reflective underlying layer and exposed to monochromatic radiation (e.g., deep ultraviolet (UV) light), standing waves may be produced within the photoresist layer. In such a situation, the reflected light interferes with the incident light and causes a periodic variation in light intensity within the photoresist layer in the vertical direction. Standing-wave effects are usually more pronounced at the deep UV wavelengths used in modern steppers than at shorter wavelengths because many commonly used materials are more reflective at deep UV wavelengths. The use of monochromatic light, as contrasted with polychromatic (e.g., white) light, also contributes to these effects because resonance is more easily induced in monochromatic light. The existence of standing waves in the photoresist layer during exposure causes roughness in the vertical walls formed when the photoresist layer is developed, which translates into variations in line widths, spacing and other critical dimensions.
One technique helpful in achieving the necessary dimensional accuracy is the use of an antireflective coating (ARC). An ARC""s optical characteristics are chosen to minimize reflections occurring at interlayer interfaces. The ARC""s absorptive index is such that the amount of monochromatic light transmitted in either direction is minimized, thus attenuating both transmitted incident light and reflections thereof. The ARC""s refractive and reflective indexes are fixed at values that cause any reflections, which might still occur, to be cancelled by incident light. This cancellation is accomplished by ensuring that reflected light is 180xc2x0 (or 540xc2x0 or another odd multiple of 180xc2x0 ) out-of-phase with respect to the incident light.
FIG. 1A illustrates another phenomenon often encountered in photolithography, known as footing. In a traditional photolithographic process, a layer 110, which is to be patterned, is deposited or grown on a substrate 120. In a traditional patterning process, a photoresist layer 130 is first deposited on layer 110. Photoresist layer 130 is then developed (i.e., patterned). This pattern is exemplified in FIG. 1A by a gap 140. Once photoresist layer 130 has been developed, the exposed areas of layer 110 may then be subjected to further processing, such as doping, etching or the like.
As is illustrated in FIG. 1A, after photoresist layer 130 is patterned, residual photoresist material may remain in junction areas 150 and 160. This residue, or footing, can cause variations in line width. Footing is underexposed photoresist material, which may remain at the foot of the vertical walls that are formed during the developing of photoresist layer 130. Footing is caused by the existence of amino groups (NH4+) at the surface of layer 110, and therefore is related to the amount of nitrogen contained in layer 110. Amino groups are slightly basic, and can form bonds with the photoresist material (which is slightly acidic) at a bottom portion of photoresist layer 130. When this occurs, the affected photoresist material is desensitized to radiant energy. Given this reduced photosensitivity, the bottom portion of photoresist layer 130 resists developing completely, and so may remain after the photoresist layer is developed. Some desensitized areas, such as areas in the center of gaps and large open areas, can be fully exposed by simply increasing the exposure""s duration.
However, an exposure of longer duration may not be effective in exposing desensitized photoresist material in areas such as junction areas 150 and 160. Radiant energy, after passing through an opening, will vary in intensity with the angle from the opening""s centerline. On average, the radiant energy""s intensity falls as the angle from the opening""s centerline increases, relative to the intensity maximum that exists at the opening""s centerline. This is in accordance with Young""s theory, which predicts this type of diffraction phenomenon. Thus, a longer-duration exposure may alter the resulting line width, but does not avoid the formation of footing.
The opening at the top of a gap, such as gap 140, may create such variations in intensity""within the gap. Because it is at an angle from the opening""s centerline (i.e., from the center of gap 140), the photoresist in junction areas 150 and 160 receives less radiant energy due to the optical mechanics of gap 140, even though the photoresist in the center of gap 140 is fully exposed. This, in turn, may result in the photoresist in junction areas 150 and 160 not being fully developed. During the subsequent patterning of layer 110, this residual material protects portions of layer 110, thereby causing the resulting line widths to deviate from intended dimensions.
The preceding description of footing is given with respect to a positive photoresist. As previously noted, photoresist is an organic compound, the solubility of which changes when exposed to certain wavelengths of radiant energy. The regions in the photoresist exposed to light become either more soluble or less soluble in a solvent called a developer. When the exposed regions become more soluble, a positive image of the mask is defined in the photoresist. This is known as a positive photoresist. If the irradiated regions become less soluble in the developer, while the nonirradiated regions remain soluble, a negative image of the mask is defined in the photoresist. This is known as a negative photoresist. A negative photoresist is believed to suffer from a similar, but opposite effect, where material is etched away from underneath portions of exposed resist, creating xe2x80x9cnegative footing.xe2x80x9d This, too, is a problem that needs to be addressed.
Yet another phenomenon encountered in photolithography is the varying etch selectively exhibited by various materials with respect to the etchants used in the patterning of layer 110. Etch selectivity indicates a material""s reactivity with respect to a given etchant in relationship to that of another material. Etch selectivity is usually denoted by a ratio of the etch rate of one material to that of another, and is usually taken with respect to the material of the layer being etched. High etch selectivity is therefore often desirable because, ideally, an etchant should selectively etch only the intended areas of the layer being patterned and not erode other structures that may already exist on the substrate being processed.
High etch selectivity is desirable in a photoresist layer because it translates into improved accuracy when transferring a mask pattern to the underlying layer. If a photoresist layer""s etch selectivity is low, the etching operation may remove not only the exposed portions of the layer being patterned, but also portions of the photoresist layer. While the removal of some photoresist material during etching is normal, extremely low etch selectivity may cause the photoresist layer to be etched through (or back, away from the exposed areas of the layer being patterned) to the point that portions of the layer being patterned, which should have been protected, are also exposed to the etchant.
The phenomenon referred to herein as photoresist etch-back is illustrated in FIG. 1B by the difference between an intended profile 270 and the actual profile of a gap 245. Because the etch selectivity of photoresist layer 220 is low, photoresist layer 220 is etched back from gap 245 during the patterning of an underlying layer 230. Without a layer of photoresist material to protect it, the additional portion of underlying layer 230 encompassed by intended profile 270 is etched away, along with the portions of underlying layer 230 originally intended for removal.
The phenomenon referred to herein as etch-through is illustrated in FIG. 1B by an etch-through 260. Etch-through occurs when photoresist layer 220, due to its low etch selectivity, is substantially etched away in a given area during the patterning of underlying layer 230. Such over-etching may cause undesirable variations in surface topology and device characteristics, and may even render devices thus fabricated inoperable. Although a thicker photoresist layer may be employed to reduce over-etching, thicker photoresist layers may require longer development times. Also, even if the photoresist layer is applied in greater thicknesses, over-etching may still occur if the photoresist layer is not applied evenly.
It is therefore desirable to provide a technique that reduces the footing experienced by a photoresist layer used in patterning a thin film in order to improve the accuracy of the patterning process. Additionally, it is desirable to ensure the protection of unexposed areas of the layer being patterned. However, such techniques should not interfere with desirable optical qualities possessed by an associated ARC layer.
The present invention addresses these requirements by providing an apparatus and a process for depositing a low nitrogen content layer to reduce footing experienced in a subsequently applied photoresist layer, without substantially adversely altering the optical qualities of an associated antireflective layer (i.e., antireflective coating, or ARC).
According to the method of the present invention, a low nitrogen content layer is deposited over the upper surface of an ARC to reduce the number of amino groups presented to a subsequently applied photoresist layer, thereby reducing footing experienced in the photoresist layer. The method of the present invention begins by depositing an ARC, which is preferably a layer of silicon oxynitride (SiOXNY) or silicon nitride (SiNX), over a substrate. The ARC is deposited to reduce the reflection and refraction of incident radiant energy occurring in a subsequently applied photoresist layer. Next, a low nitrogen content layer, such as a silicon oxide layer, is deposited. This layer is referred to herein as a capping layer. The capping layer passivates amino groups that may exist at the upper surface of the ARC, thereby improving the accuracy with which the substrate is patterned. The capping layer""s thickness is selected so that it does not substantially adversely alter the ARC""s optical qualities. The photoresist layer is then deposited over the capping layer, after which a traditional patterning process is followed, including developing the photoresist, patterning the substrate, and removing the remaining photoresist, capping layer and ARC material after the substrate has been patterned.
In one embodiment, for example, radiant energy having a wavelength of 248 nm is used. This mandates a capping layer that is preferably between about 50 xc3x85 and 150 xc3x85 in thickness. However, the use of radiant energy sources with different wavelengths may require the use of different thickness ranges. Moreover, the materials used to form the substrate, the ARC and the photoresist layer, along with other parameters, will affect the capping layer thickness selected.
In another embodiment, the present invention is used both to reduce footing in a photoresist layer and to improve pattern transfer from a mask to a substrate. Such a layer is referred to as a hardmask. A hardmask is deposited over an ARC, preferably to between about 700 xc3x85 and 1100 xc3x85 in thickness, although greater thicknesses may be used as long as the ARC""s optical qualities are not substantially adversely altered. These greater thicknesses are approximately equal to the thickness of a corresponding capping layer plus a whole multiple of about 800 xc3x85. A hardmask helps to protect the substrate during patterning operations by providing an additional thickness of protective material and, in some cases, a layer having a high etch selectivity, without substantially adversely altering the ARC""s optical qualities. This addresses over-etching of the photoresist layer via phenomena such as etch-back (the etching of material back from open areas) and etch-through (the etching of photoresist material exposing the underlying layer). A hardmask according to the present invention is thus capable of providing more accurate fabrication of structures by muting the effects caused by the excessive erosion of photoresist material during the patterning process.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.